“disqus_shortname” tells Disqus which website account (called a forum on Disqus) this system belongs to.

“disqus_identifier” tells Disqus how to uniquely identify the current page.

“disqus_url” tells Disqus the location of the page for permalinking purposes.

The “forum shortname” is required. For example, mine is “xahlee”. The other 2 are optional.

The “disqus_identifier” is important if there's the possibility that you might move the page in the future. For example, changing the URL, or moving to a new domain. It should be unique across all pages in your current account.